In a typical cellular communication network, the network determines the transmission parameters for the downlink and uplink transmission parameters based on received channel state information (CSI) report(s) and received signal quality, respectively. The transmission parameters include the modulation and coding scheme (MCS), transmit power, transmission mode and number of transmission layers, etc.
In Third Generation Partnership Project Long-Term Evolution (3GPP LTE), the MCS is typically decided by the number of information bits (or physical layer payload) that can fit in a set of scheduled physical resource block (PRB) pairs such that the eventual packet transmission is successfully received at the destination with a predefined target probability. The packet error rate depends on the resulting code rate for the transmission which is defined as the ratio of the information bits to the number of raw bits that are transmitted over the channel. The number of raw bits allocated in the transmission is dependent on the size of the PRB set and the available resource elements (REs) available for data transmission in that PRB set.
As defined in Third Generation Partnership Project Technical Specification (3GPP TS) 36.213 version 12.4.0 (“3GPP TS 36.213”), section 7.1.7, the size of the transport block size (TBS) or physical layer payload size, or the number of information bits that are transmitted is determined using TBS tables defined in 3GPP TS 36.213 based on the PRB-pair assignment and the MCS decided for that transmission. The MCS index and the PRB-pairs assigned are sent to the wireless device in the downlink control information (DCI) included in the physical downlink control channel (PDCCH). The wireless device derives the TBS size from these indices. Therefore the TBS defined in section 7.1.7 of 3GPP TS 36.213 forms the basis of control information exchange between the wireless device and the network. Here PRB-pair, as defined 3GPP TS 36.211 consists of two PRBs in each slot of the LTE subframe. For example, in the case of a LTE normal cyclic prefix (CP) subframe (as opposed to an extended CP subframe), a PRB-Pair consists of 12 subcarriers in 14 OFDM symbols. These TBS tables defined in 3GPP TS36.213 serve as a basis for serval existing methods for link adaptation. In particular, link adaptation based on these TBS tables is described as follows. A wireless device is informed about the resource allocation by transmitting a Physical Downlink Control Channel (PDCCH) (using one of the Downlink Control Information (DCI) formats for example DCI-1A or DCI-2A, etc. as further described in Section 5.3.3.1 of 3GPP TS 36.213) as follows:
1. Modulation & Coding Scheme (MCS) using 5 bits to represent a value between 0 and 31. MCS indices are defined in Section 7.1.7.1 of 3GPP TS 36.213.
2. Resources in terms of PRB pairs using one of the resource allocation types 0, 1 or 2 as described in 3GPP TS 36.213.
The wireless device determines the Transport Block Size (TBS) based on the MCS and the number of PRB pairs provided in the PDCCH. The wireless device's determination of the TBS follows Section 7.1.7.2 of 3GPP TS 36.213. For example, as illustrated in Table 7.1.7.2.1-1 of 3GPP TS 36.213, the TBS can be derived with the help of two indices: I_TBS and N_PRB, where I_TBS can be derived from the MCS index and N_PRB represents the number of PRB pairs assigned to the wireless device.
Typically, a node such as an evolved NodeB (eNB) decides the MCS and N_PRB based on a target block error rate (BLER). To achieve this, the node uses the Channel Quality Indicator (CQI) feedback from the wireless device. Coding rate for a given MCS is computed from the number of Physical Resource Block (PRB) pairs assigned for an allocation. This relation of coding gain for PRB pair assignment is implicitly defined by Table 7.1.7.2.1-1 in 3GPP TS 36.213. For example, for TBS index, ITBS, of 15 and number of PRBs, NPRB, of 20, the modulation will be QAM16 and TBS=6200. The resulting code rate is (6200+CRC)/(REs available for data or PDSCH*bits per RE)=0.5939, when the resource elements (REs) available for data or PDSCH are 2640. The cyclic redundancy check, CRC=72 is computed based on the TBS as outlined in Section 5.1.1 of 3GPP TS 36.212. The node can estimate the BLER based on the derived coding rate and the CQI feedback. MCS and PRBs are selected such that this combination of coding rate & CQI meets the required BLER target.
Further, in general, the MCS is selected such that the transmission is successful with a target error rate for a given channel quality. As defined in 3GPP TS 36.213, the number of information bits, i.e. TBS, that can be sent is implicitly decided based on the MCS and PRBs assigned for the transmission. In this case, from the link quality and the minimum number of PRBs that can successfully transmit a predetermined number of information bits with a target packet error rate can be decided. The minimization of PRBs is derived over the available set of MCSs. This procedure is described as follows:
1. Packet error rate of a selected MCS for channel quality should be decided with code rate limitations imposed by the table 7.1.7 of 3GPP TS 36.213.
2. The code rate of the MCSs varies based on the number of PRBs assigned for the transmissions.
3. When there is more than one code word, MCS has to be decided independently for each code word (based on the channel quality of each code word) with the same number of PRBs.
An existing method to decide MCS is to have Chanel Quality Indicator (CQI) values which can satisfy a target error rate for the all MCSs for different channel conditions and decide the MCS by comparing the received or measured channel quality with the reference channel quality. However, this heuristic based link adaptation based on the TBS table defined in 3GPP TS 36.213 can become computationally inefficient, and non-optimal in performance for complex transmission scenarios. In particular, this existing link adaptation method is a very complex process because of the varying performance of various modulations and code rates under varying channel conditions.